One process for the production of flat glass involves pouring molten glass from a melting furnace onto a bath of molten metal having a specific gravity greater than that of the glass. The glass forms a sheet which is made to move forwardly along the bath. During this movement, it is cooled and finally removed at the downstream end as a solidified strip having a definite width and thickness.
According to French Pat. No. 1,206,044 which is the basis of the float glass method, the only industrial one today, the molten glass is poured from the discharge or transfer unit of the melting furnace so that it can fall freely onto the molten metal bath. Actually the molten glass then spreads out backwards as well as laterally and forwards, the backward stream returning subsequently on the sides. This process has the advantage of systematically eliminating the glass that may have become contaminated by contact with the discharge or transfer unit which is made of refractory material. Since this glass fraction spreads out towards the outside of the marginal parts of the finished product it can be conveniently separated from the rest and removed. This process has made it possible to prepare glass strips having a thickness close to 6 mm, which is an equilibrium thickness resulting from surface stresses. These strips show good optical qualities, due to physico-chemical homogeneity and a satisfactory surface quality, for most commercial applications.
Commercial needs also require increasing quantities of glass with thicknesses smaller than the equilibrium thickness. This is so, for example, in the automotive industry. In the production of windshields, the thickness ranges from approximately 1.5 to 3.8 mm and is preferably around 2.3 mm. In the building industry, on the other hand, glass with thicknesses greater than the equilibrium thicknesses are required.
Through the use of various expedients and improvements, it has been possible to use the basic float process for the production of thin and thick glasses; to this end, a gradual stretching is effected on the bath surface so as to reduce the thickness of the glass, or on the contrary, the edges of the sheet are contained in order to obtain a thicker strip. However, as values are gradually further removed from the equilibrium thickness, in particular, starting at thicknesses of the order of a little less than 3.8 mm, it can be noted that the glass produced by the float process shows a greater optical distortion and that serious difficulties are encountered in the production of such a thin glass while maintaining adequate optical properties to satisfy the required conditions, in particular, for automobile windshields. It seems that this difficulty can be attributed to a large extent to the substantial agitation of the glass induced by the pouring method.
A process is also known in which, on the contrary, every effort is made to rapidly impart to the glass sheet poured on the bath, its final thickness and width by rapidly drawing it out in the direction of its width using continuous lateral guiding members. This process makes it possible to use shorter baths and has the advantage of greater production flexibility. Different embodiments of this process are described in the second and third additions 86,222 and 86,817 to French Pat. No. 1,378,839. Although it thus becomes possible to produce a glass strip having a thickness different from the equilibrium thickness, excellent optical quality still remains difficult to obtain and for reasons quite similar to those discussed above.
It has also been contemplated to form the glass sheet by simply flowing the molten glass over various sills of large width whether or not provided with an inclined plane descending to the tin bath. However, because of the small height of fall, the proposed solutions, as a whole, are not freed of the requirement of stretching the glass sheet on the surface of the bath as soon as its thickness is less than the equilibrium thickness. In addition, they have the disadvantage of very easily giving rise to defects such as bubbles on the lower face of the glass strip.